They won't care if they are anything like NPower, my son was in a rented house on a prepay meter, he moved out & 2 years later they sent him a bill for £640, they claimed that when the meter was changed that there was evidence that it had been tampered with & as such he had benefitted from the tampering & they had re-rated his usage to that of an average house.
They would never reveal what the tampering was, how they knew it was before my son moved in, I spend hours on the phone, writing emails & letters but they just wasn't interested (my son had moved to New Zealand by this time). They handed it over to several debt agencies, who couldn't provide the answers, in the end I told them to take him/us to court.
In fact now thinking about it, it has become statute barred in May this year.